The present invention relates to a hydrolyzed ethylene-vinyl acetate copolymer, and more particularly to a hydrolyzed ethylene-vinyl acetate copolymer having an improved moisture resistance, particularly an excellent gas impermeability even under high humidity, and capable of continuing melt-molding for a long time (this property being hereinafter referred to as long-run moldability).
Hydrolyzed ethylene-vinyl acetate copolymers are excellent in oxygen impermeability, a mechanical strength, and the like, so they are widely used in various uses such as films, sheets, containers and fibers. However, the hydrolyzed copolymers have defects that the physical properties such as the flexibility of the molded articles therefrom vary remarkably due to change of humidity and temperature of the outside, and, particularly, the gas impermeability is lowered under high humidity, since the hydrolyzed copolymers have high moisture absorption.
Also, when the hydrolyzed copolymers are continuously melt-molded for a long time, there is a case that the long-run moldability is poor, such that gels are formed in the melted hydrolyzed copolymer, the colored deteriorated hydrolyzed copolymer is accumulated at an inside of a molding machine to lower the efficiency in molding, and the obtained molded articles are deteriorated in the quality.
In order to improve the lowering of the gas impermeability under high humidity, it has been proposed, for instance, (1) to laminate a moisture resistant resin such as a polyolefin on a film or sheet of the hydrolyzed ethylene-vinyl acetate copolymer to prevent permeation of water-vapor from the outside, (2) to introduce crosslinkable groups such as silyl group into the structure of the hydrolyzed ethylene-vinyl acetate copolymer to impart moisture resistance to the hydrolyzed copolymer, or the like. On the other hand, in order to improve the long-run moldability of the hydrolyzed copolymer, it has been adopted to admix a lubricant with the copolymer on melt-molding, or the like.
In the above-mentioned improvement of the gas impermeability, however, in the former (1) has defects that laminating operation is required, and even if laminating the moisture resistant resin on the film or sheet of the hydrolyzed copolymer, it is difficult to prevent permeation of water-vapor from the edges of the film or sheet. Also, the latter (2) has defects that crosslinking or gelation of the hydrolyzed copolymer is caused easily upon the preparation or molding of the hydrolyzed copolymer, and the moldability is lowered. On the other hand, in the improvement of the long-run moldability, the properties of the hydrolyzed ethylene-vinyl acetate copolymer, for example, the adhesion to other resins on production of its multi-layer laminates, are lowered since the copolymer is admixed with the lubricant.
An object of the present invention is to provide a hydrolyzed ethylene-vinyl acetate copolymer having improved moisture resistance and long-run moldability without the above-mentioned defects.
This and other objects of the present invention will become apparent from the description hereafter.